I've completed the game as a sniper assassin and I'm playing through again on True Vault Hunter Mode. For the most part my damage is scaling up and as long as I'm in the zone getting quick critical hits I'm handling normal mobs just fine.

Bosses and mini-bosses are giving me the most trouble. Often I have to get close to spawn them and my first reaction is to hit Decepti0n and sprint away. But most of them can close almost any distance in one leap and chase me as fast as I can sprint. With about 10k/10k shields/health I can only take a few hits and have to rely on health stealing grenades and repeated sprints to try to kite the bosses. Mini-bosses are often in confined spaces that get me killed quickly.

3 Answers
3

I don't really see much options on slowing enemies down, so I'll add tips on surviving close quarters combat, and/or escaping it when playing as a Sniper (assuming you're focusing on the 'Sniping' skill tree).

See the 'Status Effects - Stunning' article at the Borderlands wiki (most of it is written for Borderlands 1, but a lot still applies to Borderlands 2). It dicusses how enemies are stunned, which should slow them down.

It greatly improves accuracy of sniper rifles when aiming from the hip. It also increases the reload speed and magazine size of sniper rifles.

This well help you to better shoot your sniper rifle without using the scope - useful as a lot of the skills in the 'Sniping' skill tree improve the efficiency of sniper rifles specifically. This will make it easier to fight or whittle down enemies while escaping close quarters combat.

Hyperion 'Singularity' grenades pulls in nearby enemies before exploding, potentially slowing them down, while dealing heavy damage. When overwhelmed, throw a grenade, while going for the nearest cover.

Torgue/Maliwan 'Nova' shields will fire elemental blasts once depleted. They could help damage or whittle down nearby enemies in close-quarters combat, making it easier to fight or escape.

Use a Relic that increases Decepti0n's cool down rate ('Proficiency' relics) or helps improve 'Fight For Your Life' time ('Tenacity' relics or Sheriff's/Deputy's badge). You will either be more likely to escape with Decepti0n when you need to use it, or have more time to get a second wind, when crippled.

Get a 'Professional' class mod. It increases reload speed and shield capacity, making you take more damage, and making it easier to fight back without using the sniper scope or a sniper rifle. It will still help if you're firing the sniper rifle unscoped and you have At 0ne with the Gun skill points. Another option is to equip a 'Spy' class mod (faster Decepti0n cooldown rate) or a 'Survivor' class mod (increase max health and health regeneration).

When playing co-op, let you non-sniper allies be at the front lines, and just snipe enemies from a safe distance.

Be wary of places where enemies might spawn. Also, have a strong backup non-sniper gun ready, or use your Sniper rifle unscoped if you have At 0ne with the Gun skill points, when fighting or escaping close quarters combat.

You may need to use your environment in your favor, like trying to kite them around pillars or other cover, as a general tip.
To slow down most robots you can shoot their legs off, that wil help in the regard of "slowing down", as they need to crawl.
With biological enemies there's nothing you can do but pump out damage, until they slow down because of their injuries at around 10% - 20% health.I didn't notice any other way to slow enemies down, than the ones mentioned.

This is a pretty cheesy way to do it, and it doesn't work for all mobs, but Singularity grenades can help. Throw one to the ground as you're running away and by the time they leap at you, your grenade will pull them in. Doesn't always work, but it effectively helps in kiting. Unless you want to stick to your Life Leech grenades.